Paleomagnetic secular field variation and rock magnetism of some early Holocene (<9900BP) postglacial lacustrine sediments near Thunder Bay, northwestern Ontario, Canada.

摘要

Paleosecular magnetic changes through a 1.5m section of post-glacial lacustrine deposits are strongly influenced by mineralogy and differential compaction. The sediments chiefly comprise clay and, in the lower one third part of the section, rhythmites which vary from ∼2mm to ∼5cm in thickness. 125 paleomagnetic specimens were collected in total from the 1.5m section. Anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanent magnetization identify a magnetic fabric with maximum susceptibility parallel to the bedding plane of the sediments and with a preferred northwest-southeast axis, probably indicating paleo-current alignment. Minimum susceptibility represents the pole to bedding, due to grain alignment.;Incremental alternating field demagnetization of clay, in 10 to 17 stages, successfully isolated three distinct remanence components, with different directions. A weak and spurious component that is due to handling magnetization, for example, was removed in early demagnetization steps. The two remaining components, isolated by further demagnetization, have geological significance. Mineral magnetic properties as well as differential compaction are responsible for the different directions of the two geologically significant magnetizations in the clay. One has intermediate coercivities (20 -- 40mT) with a Fisher mean declination = 357.4° and Fisher mean inclination = 62.0°, approaching the present Earth's magnetic field (declination=356.7° and inclination=74.7°). The other has harder coercivities (>40mT) and represents a stable geological magnetization. The silt has a single component of magnetization carried by magnetite. Laboratory re-deposition of clay in large (125cm3) cubes accurately records the direction of the laboratory magnetic field; thus major inclination shallowing in natural sediments occurred subsequent to acquisition of original DRM.;A magnetostratigraphic record of the hard coercivity component over the 1.5 meter section reveals paleosecular variation of the Earth's magnetic field, with some complications due to compaction. A single westward drifting loop is recorded in the sediments, representing a time-period of circa 600 -- 2400 years, with declinations varying from 199.8° to 119.9° (Fisher mean=005.2°) and inclinations varying from -2.4° to 77.4° (Fisher mean=50.9°). In comparison the inclination of the geocentric axial dipole (GAD) for the study location is +66°.;Incremental acquisition and demagnetization of isothermal and anhysteretic magnetizations and the orthogonal three-axis test indicate that the sediments contain two magnetic phases with different coercivities. Magnetic hysteresis measurements (clays n=226; silts n=37) show that clay is dominated by single domain magnetite and hematite (means of Ms=58.47+/-9.22Am 2, Mrs=17.12+/-27.22Am2, Hc=21.09+/-7.69mT, Hcr=62.04+/-4.09mT) whereas silt is dominated by pseudo-single domain and single domain magnetite (means of Ms=681.0+/-395.9Am 2, Mrs=163.2+/-84.75Am2, Hc=26.07+/-2.94mT, Hcr=56.08+/-3.17mT). The silt is dominated by magnetite, whereas the clay carries both hematite and magnetite.